Visa options for academic research in France?

Hi everyone -- I'm new here, and would appreciate some help!

I’m currently a graduate student in the US, and I’m trying to figure out ways to spend next year in France as I work on my dissertation.
So, due to various outside circumstances, I missed the deadline to participate in a grad student exchange program.  However, I’ve found that it might be possible for me to tag along with a program that’s aimed more at undergrads, and just take a masters course per semester while I work on my diss., and that way I’d also be able to get a student visa, etc.
However: I’m not sure if that’s the best idea, and I think I might prefer to be more of a free agent and just have time to focus on my own research.  That said, I’m not so sure how to handle the visa process for that, since I wouldn’t be attached to a particular program/university in France. 

So basically, my question is: would it be a good idea for an American graduate student to just opt for a long-stay tourist visa?  Is there any better option if what I mainly want to do is stay in France and do my own research, rather than attend classes (though I am technically still a student)?

Any thoughts would be much appreciated.  My profs, peers, and our study abroad office (which caters mainly to undergrads) have unfortunately not been able to offer much advice.

(This would be for a stay of approx. 9 months total.)

Thanks!